It has previously been proposed to control the position of a throttle or other similar fuel control element by a servo motor, see, for example, the referenced U.S. Pat. No. 4,495,454. The position of a throttle flap, to a predetermined opening cross section of the induction pipe, can be controlled by suitably energizing a servo or other positioning motor based on a command signal. The command signal can be in digital form, that is, can be a digital command word which is transformed into an analog command value in a D/A converter. The command signal is then applied to a comparator which compares the command signal with an actual position signal derived from a position transducer and, for example, coupled to the servo motor or other positioning motor and/or to the positioned element, for example the throttle or other fuel supply controller. The actual position signal derived from the positioning transducer thus will be a measure of fuel being supplied and, under given operating conditions, of a certain speed of the motor. This actual signal value is compared in the comparator with the command signal value. The D/A converter usually is controlled, in turn, by a register or counter which, of course, may form part of a microcomputer or an equivalent functional stage within a microcomputer, or externally thereof.
The resolution which is obtainable by a D/A converter depends on the resolution of the digital command word. In order to obtain higher resolution, a D/A converter is needed which is capable of handling digital command words having a high number of bits. This, in turn, requires a control counter for the D/A converter, in, or externally of the microcomputer. Controlling the D/A converter with command words having a large number of bits is expensive and requires substantial circuit components; the complexity of such circuits may lead, additionally, to an increased possibility of malfunction. Further, in many automotive control systems it is customary to use command words of eight bits and, to obtain higher resolution, it is necessary to include additional circuitry if a higher resolution of command words is needed.